To resolve this I tried changing SYS user password so many times, recreated password file and copied that to standby, but nothing worked out.

Finally I resolved this by checking following parameters which are the culprtis in my case

REMOTE_LOGIN_PASSWORD

REMOTE_OS_AUTHENT

By default the above parameters should set to EXCLUSIVE and FALSE values respectively. But due to some reason, they are set to NONE and TRUE in my case. I changed them to EXCLUSIVE and FALSE then its resolved

Note: changing those parameter values need a restart of database.

So, when ever you face similar issue of invalid username/password during standby creation, check the above parameters and change the values (if needed) and try.

As always don’t try this directly on a prod env, instead first test it…. HAPPY LEARNING